Sky & Telescope - April 2019 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion

Frank's Four-Mirror Binoscopes
the ﬁrst mention I
can ﬁnd of it is in
Albert G. Ingalls's
1932 book Amateur
Telescope Making
- but for some
reason the idea has
never really caught
on. It should have,
though, because
the design is
simplicity itself:
Simply lay two
Newtonian scopes
Frank shows off "Popeye"
side by side and
at the Oregon Star Party
offset them both
telescope walkabout.
horizontally and
vertically so the
light path of one crosses through the
IT'S COMMON KNOWLEDGE in the
light path of the other. When the scopes
ATM world that a reﬂecting binocular
are oriented properly, their light cones
telescope requires six mirrors. You need
shine directly into an observer's eyes
the primaries to gather light and bring
without any further ado.
it to a focus, you need secondaries to
It's that "oriented properly" part that
bounce that light off to the side, and
probably scares off a lot of people, but
you need tertiaries to make the two
compared to collimating six mirrors on
light cones parallel again and aim them
a more traditional binocular scope, it's
into your eyes.
Like most common knowledge, that's not even in the same league.
There are a couple of ways to do it:
not necessarily so. There's a way to
You can strap two separate OTAs side by
make a binocular scope with just four
side, or you can build both OTAs into a
mirrors, and Oregon ATM Frank Szczsingle framework. Frank has gone with
epanski has become a master at it.
the second method.
The idea has been around a while -

Secondary
mirrors

Eyepieces

72

A PR I L 2 019 * SK Y & TELESCOPE

Primary
mirrors

tq The light cone
from the far OTA
crosses through the
near one, eliminating the need for
tertiary mirrors.

One of the ﬁrst things you notice
when you look down the front of one of
Frank's binoscopes is that the secondary
mirrors are different sizes. The one in
the OTA that's farther from the eyepiece
must be larger than the nearer one,
since it's intercepting the light cone
several inches more distant from the
eyepiece. That creates a greater obstruction, but the effect is barely noticeable
to the eye.
The longer distance between eyepiece
and secondary on the more distant
OTA means that the primary must be
mounted farther forward. Frank puts
each primary on its own bulkhead. His
scopes are designed to come apart for
transport, so there are four bulkheads,
all cut and drilled as a single sandwich
so everything is perfectly aligned. By
doing that and being very careful about
centering each optical element, Frank
doesn't need any fancy mounting for
the primaries. They have the traditional
tilt capability for collimation; no sideto-side adjustments are necessary. To
ﬁne-adjust image merging, he tilts just
one of the primaries with extended collimation screws that he can reach from
the eyepiece.
People's eyes vary in separation. To
adjust the interpupillary distance, Frank
mounts the secondary mirror of the
nearer OTA on a sliding plate that also
contains the focuser. The entire works
slides up and down the tube, moving the left eyepiece closer to or away
from the right one. Once the correct
spacing is achieved, the left eyepiece is
refocused. Frank's
latest binoscope, a
10-inch f/5, uses
2″ reverse-Crayford
focusers, but other
designs simply use
snug tubes, and you
slide the eyepieces

STA R PA RT Y: JERRY OLTION; ILLUSTR ATION: G REGG DINDER M A N / S&T

Tertiaries? Who needs tertiaries?
Sky & Telescope - April 2019

Table of Contents for the Digital Edition of Sky & Telescope - April 2019
